Electrolytic polishing solution



2,762,766 Patented Sept. 11, 1956 United States Patent 2,762,7616 H ELECTROLYTIC POLISHING SOLUTION Bo-Shin R0, Tokyo, SJapalgi, assigiglri'fto Khe-Beng 'Chiong, an ego,

N Drawing. Application April 15, 1952, Serial No. 282,485

3 Claims. (Cl. 204-1405) This invention relates to electrolytic polishing of steels, more particularly to a process, for electrolytic polishing of high speed steels in an acid bath.

One of the objects of this invention is to provide a rapid, direct, reliable process without the use of manual labor for the polishing of high speed steels.

Still another object of my invention is the provision of a process that will allow the continuous treatment of steel parts so as to produce a high polished lustre on the surface thereof. The characteristic feature of this invention is a bath consisting essentially of nitric acid containing a small amount of hydrofluoric acid to which may be added some potassium dichromate.

High speed steels containing tungsten, molybdenum, cobalt, chromium, nickel or some of these alloying metals, the balance being iron, have heretofore resisted efforts to produce a high polish except by mechanical methods. Due to the fact that these steels are very hard, it renders such operations tedious, difficult and expensive, requiring skilled labor and hand operation. Stainless steel usually contains nickel, chromium and other ingredients but does not usually contain tungsten. It is highly desirable to polish all steel parts. I have found that such a polish is obtained by treating the steel parts with a solution that forms a diflicultly soluble salt, or compound, on the surface thereof, which compound is more easily removed I from the high spots on the surface and therefore reacts so as to produce a level, even surface. After treatment for a comparatively very short length of time a high polish of a mirror-like nature producing a characteristic sheen is obtained. This short period of treatment allows the invention to be practiced as a continuous process.

A suitable electrolytic bath contains:

Parts Nitric acid 100 Hydrofluoric acid 1 to 10 Potassium dichromate 0 to 2 The nitric acid is highly concentrated having a density of 1.52. The hydrofluoric acid is preferably a 50% concentration. The potassium dichromate is added in the form of solid crystalline material. This latter ingredient is not a necessary part of this invention but is added to the preferred solution. It is apparent that this solution contains little or no water and this is advantageous, because the acid concentration is very high and must be maintained in this condition.

Due to the fact that this bath is very concentrated the temperature must be maintained within the range of 5 C. to 20 C. If it exceeds about 20 C. N02 gas is generated, which gas is not only harmful to personnel but depletes the nitric acid concentration in the bath with deleterious results. If the temperature is below 5 C. the reaction is retarded to such an extent that a longer time is required.

A difference of potential of 7 volts is usually employed, at which voltage a current density of 120 amperes per dm. is found to give satisfactory results. Under these conditions a polish is obtained in 60 seconds, such polish giving a high lustre and a smooth non pitted surface.

It has been found that an oxide film frequently is formed on the surface of steel parts which makes it diificult to obtain an even surface by ordinary polishing methods. However, this present process enables an oxidizer to be added as required so as to permit the removal of portions of this surface. I have discovered that the convex parts of this film are less stable than the concave parts and thus allow the removal of the high places by means of the acid more readily than the low places, thereby producing an even surface having a mirror-like finish and a high gloss. This removal of metal may be controlled by shortening or lengthening the time of immersion within the bath to produce an article having a predetermined amount of metal removed from the surface thereof.

It has been found that tungsten compounds which are incorporated in high speed steel Will dissolve in the solution set forth above, which consists primarily of concentrated nitric acid and hydrofluoric acid in the ratio of 100 to 110, said solution containing little or no water.

Example High speed steel containing tungsten is immersed in a bath containing:

at a temperature of 15 C. and a difference of potential of 20 volts, at current denity of 120 amperes per (1111. with a cathode plate of stainless steel 21 high polish was obtained in 60 seconds.

Upon removal of the steel parts from this bath they are Washed and dried, completing the treatment.

Direct current is preferably used in the above electrolytic process, which allows the use of all of the current in the electrolytic polishing of the metal. In this respect this process is superior to processes designed to use alternating current where only the positive phase of the current is of benefit, the negative phase producing no noticeable results.

It will thus be seen that this invention provides a process for the polishing of high speed steel, which has heretofore been accomplished with great difficulty, and of course it may also be applied to stainless steel which is more easily handled and more easily polished. The example set forth in detail above, is only one illustration of the application of this invention, as defined by the ranges in the proportions of the ingredients, and defines the invention broadly. Many embodiments of this invention are possible, and it is Within the scope and intent of this invention to use all of these embodiments.

What is claimed is:

1. A method of electropolishing high speed steel which comprises contacting the steel with a solution consisting of 100 parts concentrated nitric acid s. g. 1.52, 4 parts hydrofluoric acid containing not less than hydrofluoric acid and 2 parts potassium dichromate, at a temperature of 15 C. and at a potential of 10 volts with a current density of 120 amperes per dm. for seconds, removing the parts and Washing the same.

2. A method of electropolishing stainless steel which comprises the steps of contacting the steel parts with a solution consisting of parts concentrated nitric acid s. g. 1.52, 4 parts hydrofluoric acid containing not less than 50% hydrofluoric acid and 2 parts potassium dichromate, at a temperature of 16 C. and passing a current at a potential of 10 volts with a current density of amperes per drn. from the parts to an electrode of stainless steel for 20 seconds, removing the stainless steel parts and Washing the same to remove the acid.

3. A process for electro-polishing stainless steel which comprises immersing the product to be polished in a bath containing not more than 5% water and consisting essentially of 100 parts of concentrated nitric acid and 1 to 10 parts of hydrofluoric acid, having not less than 50% hydrofluoric acid, and about 2 parts of potassium dichrornate.

References Cited in the file of this patent UNITED STATES PATENTS Feild Ian. 4, 1949 Blaut et al Apr. 26, 1938 Hesch June 2, 1953 FOREIGN PATENTS Great Britain Dec. 4, 1940 

1. A METHOD OF ELECTROPOLISHING HIGH SPEED STEEL WHICH COMPRISES CONTACTING THE STEEL WITH A SOLUTION CONSISTING OF 100 PARTS CONCENTRATED NITRIC ACID S. G. 1.52, 4 PARTS HYDROFLUORIC ACID CONTAINING NOT LESS THAN 50% HYDROFLUORIC ACID AND 2 PARTS POTASSIUM DICHROMATE, AT A TEMPPERATURE OF 150* C. AND AT A POTENTIAL OF 10 VOLTS WITH A CURRENT DENSITY OF 120 AMPERES PER DM.2 FOR 60 SECONDS, REMOVING THE PARTS AND WASHING THE SAME. 